Waveguides

Abstract

A waveguide 112 includes a substrate of material 134 having optical layers 148, 152 applied to two external surfaces 146, 150 . This reduces the critical angle c 5 of the substrate of material 134 to provide greater interaction between image bearing light following a light path 140 and a grating element 142 and/or a greater total field of view 160 , when compared to the total field of view 132 of a prior art waveguide 110 , that is capable of being transmitted by the waveguide 112 . Such a waveguide 112 can be used in a projection display.

The invention claimed is: 
     
       1. A waveguide for a projection display for displaying an image to an observer, comprising:
 a substrate of light transmissive material having two opposed substantially parallel faces, the substrate of light transmissive material being arranged to transmit light within the substrate of light transmissive material, between the parallel faces, under total internal reflection, the substrate of light transmissive material having a critical angle required for total internal reflection and the substrate of light transmissive material being arranged to allow the observer to view a forward scene through the substrate of light transmissive material; 
 an output grating element arranged within the substrate of light transmissive material to diffract a portion of light incident thereon out of the substrate of light transmissive to allow light exiting the substrate of light transmissive material to be viewed by the observer overlaid on the forward scene viewed through the substrate of light transmissive material and to allow a remainder of the light to be transmitted within the substrate of light transmissive material under total internal reflection; and 
 an optical layer arranged on each of the parallel faces, the optical layer being arranged to reduce the critical angle required for total internal reflection of light within the substrate of light transmissive material to increase a number of interactions between light transmitted within the substrate and the output grating element. 
 
     
     
       2. A waveguide, as claimed in  claim 1 , comprising means for inputting light into the substrate at angles of incidence relative to the substantially parallel faces which are between a critical angle of the substrate without the optical layer and a reduced critical angle of the substrate with the optical layer. 
     
     
       3. A waveguide, as claimed in  claim 1 , wherein the optical layer is selected to reduce Fresnel reflectance of light at the faces of the substrate having an angle of incidence which is between the critical angle of the substrate with the optical layer and a normal axis when light is diffracted out of the substrate by the output grating element. 
     
     
       4. A waveguide, as claimed in  claim 1 , wherein the optical layer comprises a plurality of layers in which each alternate layer of the plurality of layers has a higher or lower refractive index with respect to an adjacent layer of the plurality of layers. 
     
     
       5. A waveguide, as claimed in  claim 4 , wherein a layer of the plurality of layers adjacent to a lower refractive index layer comprises TiO 2 . 
     
     
       6. A projection display, for displaying an image to an observer, comprising:
 a first waveguide and a second waveguide both in accordance with the waveguide as claimed in  claim 1 ; 
 an image-providing light source device arranged to generate an image and to inject image bearing light into the first waveguide; a first said output grating element associated with the first waveguide arranged to direct the image bearing light internally along the first waveguide under total internal reflection and via which the image bearing light is outputted from the first waveguide; 
 an input grating element arranged to receive the image bearing light from the first waveguide and to inject the image bearing light along the second waveguide under total internal reflection; and 
 a second said output grating element arranged in the second waveguide to diffract received image bearing light out of the second waveguide towards an observer; 
 wherein the optical layers of the first and second waveguides reduce the critical angle required for total internal reflection of image bearing light within the waveguides to increase the number of interactions between the light propagating within the first waveguide and the first output grating element and between the light propagating within the second waveguide and the second output grating element. 
 
     
     
       7. A projection display as claimed in  claim 6 , wherein one or more of the grating elements are holographic optical elements or semi-reflective surfaces. 
     
     
       8. A projection display, as claimed in  claim 6 , wherein the first waveguide is plate-like, the second waveguide is plate-like and the first and second waveguides are arranged substantially parallel to one another. 
     
     
       9. A projection display, as claimed in  claim 6 , wherein the first waveguide and the second waveguide are arranged substantially in the same plane. 
     
     
       10. A projection display, as claimed in  claim 8 , wherein the first waveguide and the second waveguide are formed within a single piece of material. 
     
     
       11. A projection display, as claimed in  claim 6 , wherein the first waveguide is rod-like, the second waveguide is plate-like and the first and second waveguide are arranged such the image bearing light exits the first waveguide along a longitudinal axis of the first waveguide and enters the second waveguide to propagate along a propagation axis of the second waveguide. 
     
     
       12. A head up display, or helmet mounted display, or head mounted display including a waveguide as claimed in  claim 1 . 
     
     
       13. A head up display, or helmet mounted display, or head mounted display including a projection display as claimed in  claim 6 . 
     
     
       14. A waveguide, as claimed in  claim 4 , wherein a layer of the plurality of layers adjacent to a higher refractive index layer comprises MgF 2 .